1. Field of the Invention
The present invention relates to a zirconia ceramic powder and a tetragonal zirconia-alumina composite using the ceramic powder which have a low temperature degradation resistance as well as a high strength and high toughness, and to a method of a preparation for the same, and in detail, to a tetragonal zirconia ceramic powder having a triangle composition range formed of three composition points of 92 mol % ZrO2-4 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), 89 mol % ZrO2-7 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), 86 mol % ZrO2-7 mol % Y2O3-7 mol % Nb2O5 (or Ta2O5) in which the composition ratio of the tetragonal zirconia is in the ternary system of ZrO2xe2x80x94Y2O3xe2x80x94Nb2O5, or a zirconia/alumina composite comprising the above-described tetragonal zirconia and 5xcx9c50% (v/v) alumina and a manufacturing method of the same, which has a low temperature degradation resistance as well as a high toughness and high strength.
2. Description of the Background Art
A pure zirconia has three polymorphic forms of a monoclinic phase, tetragonal phase and cubic phase based on a temperature under an atmospheric pressure. In the case the pure zirconia, it is known to have a cubic phase from a melting point of zirconia to 2370xc2x0 C. when cooling the same at a high temperature, a tetragonal phase from about 2370xc2x0 C. to about 1170xc2x0 C., and a monoclinic phase below 1170xc2x0 C. When cooling a high temperature tetragonal phase above 1170xc2x0 C., the phase is transformed to the monoclinic phase at 950xc2x0 C., so that a volume is expanded by 3xcx9c5% for thereby forming cracks over the entire portions of a sintered body. In order to prevent a martensitic phase transformation, an oxide such as MgO, CaO, Y2O3, CeO2, etc. is added as a stabilizing agent for thereby stabilizing a cubic phase or tetragonal phase which is stable in high temperature. The stabilized Yxe2x80x94TZP(YO-stabilized Tetragonal Zirconia Polycrystalline) in which all ceramic grains are formed in the tetragonal phases by sintering under a condition that the tetragonal phase is stable by adding Y2O3 is a material having a high strength above 1000 MPa by decreasing the flaw size due to a crystal control effect. However, when this material is exposed at 100xcx9c400xc2x0 C. for a long time, the tetragonal phase is transformed to the monoclinic phase for thereby causing cracks, so that the mechanical characteristics (for example, strength) of the material are degraded. In order to prevent the low temperature degradation phenomenon and strength decrease, an alumina is added into the zirconia for thereby manufacturing a composite. In the case when adding the alumina, the alumina acts to minimize the flaw size and acts as a grain-growth inhibits of the zirconia, so that the zirconia-alumina composite have a high strength compared to the zirconia monolith. Namely, since the strength of the zirconia is in reverse proportional to the grain size, the alumina added into the zirconia prevents a grain growth of the zirconia during a sintering operation for thereby increasing the strength of the tetragonal zirconia.
Various zirconia/alumina ceramics are disclosed. For example, according to the U.S. Pat. No. 4,298,385 by Claussen, it is known that a composite of alumina, zirconia and HfO2 has high toughness. However, in the U.S. Pat. No. 4,298,385, it is difficult to maintain a tetragonal phase as the amount of zirconia is increased, and the grain size of the zirconia must be maintained below 0.5 xcexcm.
According to the U.S. Pat. No. 4,316,964, a zirconia and alumina composite into which Y2O3, CeO2, La2O3, Eu2O3, etc. are added for stabilizing the tetragonal phase is disclosed by Lange. According to the U.S. Pat. No. 4,533,647, different composition of the alumina/zirconia composite is disclosed by Tien. A zirconia and HfO2 are added into the alumina into which Cr2O3 is dissolved to increase the phase transformation temperature from the tetragonal phase to the monoclinic phase for thereby increasing the toughness. However, in this patent, it was known later that the toughness was not increased.
According to the U.S. Pat. No. 4,552,852, a composite manufactured by adding a zirconia or HfO2 and glass phase into the alumina is disclosed by Manning. The thusly manufactured material has an increased thermal shock resistance.
According to the U.S. Pat. No. 4,587,225, a zirconia/alumina composite into which Y2O3 is added by Hot isostatic press method is disclosed by Tsukuma. This composite has a high strength and is sintered for a short time at a lower temperature compared to other composite.
According to the U.S. Pat. No. 4,666,467, a high strength zirconia/alumina composite is disclosed by Matsumoto. The composition of the same is formed of a zirconia 50%xcx9c98% added by 1.5xcx9c5 mol % Y2O3 and an alumina or spinel of 50%xcx9c2 weight %. According to the U.S. Pat. No. 4,659,680, a method for manufacturing a zirconia which is partially stabilized by adding yttria and implementing a secondary stabilized phase. In this method, the sintered body is quickly cooled from a temperature range of 1000xc2x0 C.-1475xc2x0 C. and is maintained for a long time at a temperature at which the zirconia is precipitated inside a cubic grain.
According to the U.S. Pat. No. 4,760,038 by Kinney et al., a composite is manufactured by adding 5xcx9c35% ZrO2 as a first additive and 0.25 through 5% TiO2 and MnO2 as a second additive and a third additive respectively using the alumina as a main component. The above-described additives are used to increase a thermal shock resistance of the ceramic composition and prevent a decrease of the strength in a high temperature.
According to the U.S. Pat. No. 4,829,028 by Ichiro, et al., an alumina/zirconia composite having an excellent mechanical strength is manufactured using a Bayer alumina or bauxite for alumina and a vedellate mineral rock for a zirconia. In addition, according to the U.S. Pat. No. 5,061,665 by Ichiro, et al., a molten body is manufactured using the alumina and zirconia and then is rapidly cooled, and at least one ceramic powder selected from the group comprising CeO2 and TiO2 having an average diameter of 0.5 through 1.5 xcexcm is added to the resultant solid material and is ground and sintered for thereby manufacturing an alumina/zirconia composite. This composite has an excellent strength and thermal shock resistance characteristic depending on the amount of ZrO2, CeO2, and TiO2.
According to the U.S. Pat. No. 5,556,816, a new zirconia among the compositions ternary system of ZrO2xe2x80x94Y2O3xe2x80x94Nb2O5 or Ta2O3, is disclosed. This zirconia has a high toughness and a phase transformation does not occur from the tetragonal phase to the monoclinic phase. However in this patent, the physical strength in which the grain size acts as one of the important factors is very low die to the large grain growth.
As described above, there are various patents and reports on the zirconia/alumina composite for the reason that the zirconia""s grain in which the martensitic phase transformation occurs enhances a lower mechanical properties of the alumina, and the alumina having a large Young""s modulus restricts the low temperature degradation of the zirconia. However, as shown in FIG. 1, adding the alumina to the tetragonal zirconia does not restrict the low temperature degradation of the tetragonal zirconia under the autoclave processing condition at a temperature of 200xc2x0 C. and 200 MPa vapor pressure. Only when heat-treating in the air at a temperature of 100xcx9c400xc2x0 C., the low temperature degradation is delayed.
The present invention is directed to a zirconia/alumina composite and a manufacturing method of the same which provide a high strength and toughness and a low temperature degradation resistance.
Accordingly, it is an object of the present invention to provide a tetragonal zirconia having a triangle composition range formed of three composition points of 92 mol % ZrO2-4 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), 89 mol % ZrO2-7 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), and 86 mol % ZrO2-7 mol % Y2O3-7 mol % Nb2O5 (or Ta2O5) in a ternary system of ZrO2xe2x80x94Y2O3xe2x80x94Nb2O5 or Ta2O3 or a zirconia/alumina ceramic powder formed of 5xcx9c50% alumina based on a volume ratio with respect to the tetragonal zirconia.
It is another object of the present invention to provide a zirconia/alumina ceramic powder manufacturing method which includes the steps of adding 5xcx9c50% alumina based on a volume ratio with respect to the tetragonal zirconia into a tetragonal zirconia ceramic powder having a composition in a triangle composition range formed of 92 mol % ZrO2-4 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), 89 mol % ZrO2-7 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), and 86 mol % ZrO2-7 mol % Y2O3-7 mol % Nb2O5 (or Ta2O5) in a ternary system of ZrO2xe2x80x94Y2O3xe2x80x94Nb2O5 (or Ta2O3) and shaping a resultant mixture and sintering a resultant shaped body at a temperature of 1300xcx9c1700xc2x0 C.
The above-described tetragonal zirconia ceramic powder or zirconia/alumina composite has the above-described composition, so that a low temperature degradation is prevented based on a high toughness and high strength. Generally, when there is moisture at an atmospheric pressure, the low temperature degradation more easily occurs. In particular, the degradation is more increased under a certain hard condition such as an autoclave. As shown in FIG. 1, adding the alumina to the tetragonal zirconia does not prevent the low temperature degradation under an autoclave processing condition at a temperature of 200xc2x0 C. and under 200 MPa vapor pressure. The low temperature degradation is only delayed by processing in the air at a temperature of 100xcx9c400xc2x0 C. for a long time. However, in the case of the tetragonal zirconia ceramic powders and the tetragonal zirconia/alumina composite using the same having the composition according to the present invention, the low temperature degradation does not occur under a certain condition at a temperature of 200xc2x0 C. and under a pressure of 200 MPa and has a high strength and high toughness. As the alumina is added, it is possible to obtain a stable tetragonal zirconia solid solution, and the toughness is increased due to a crack deflection based on the alumina. The strength is increased by decreasing the grain size. The phase transition from the tetragonal phase to the monoclinic phase is effectively controlled by adding an alumina having a higher elastic modulus compared to the zirconia. Namely, the low temperature degradation is more effectively controlled.
In order to provide a zirconia/alumina composite having the above-described characteristic, when mixing TZP powders, Y2O3 is added more than the adding amount of Nb2O5 (or Ta2O5), and the composition range is determined based on a triangle composition area formed of three composition points of 92 mol % ZrO2-4 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), 89 mol % ZrO2-7 mol % Y2O3-4 mol % Nb2O5 (or Ta2O5), and 86 mol % ZrO2O3-7 mol % Y2O3-7 mol % Nb2O5 (or Ta2O5).
The alumina among the zirconia/alumina composite is preferably has a grain size of 0.1xcx9c10 xcexcm. If the grain size of the alumina is below 0.1 xcexcm, the toughness enhancing effect by the crack deflection is decreased, and if the grain size is above 10 xcexcm, it is difficult to implement a densification of the zirconia and composite during the sintering process. FIG. 2 illustrates a strength and toughness of the tetragonal zirconia/alumina composite based on the grain size and adding amount of the alumina. The alumina of a larger grain size has a higher toughness compared to the alumina of a smaller grain size. Therefore, it is known that an alumina having a larger grain size is preferred. As shown in FIG. 2, and as described in the example 9-16 of the present invention, the adding amount of the alumina is 5xcx9c50% (v/v), preferably, 10xcx9c30(v/v) %, and is more preferably 20(v/v) %.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.